Recently, many developers have conducted intensive research into a handover method for use in a variety of communication systems. A representative example of the communication systems is an Evolved Universal Mobile Telecommunications System (E-UMTS), such that a detailed description thereof will hereinafter be described.
FIG. 1 is a structural diagram illustrating an E-UMTS network. It should be noted that the E-UMTS network of FIG. 1 can be applied to the conventional art and the present invention. The E-UMTS is an evolved version of a convention UMTS system, and is being standardized by the 3GPP (3rd Generation Partnership Project). The E-UMTS may also be considered to be a Long Term Evolution (LTE) system.
The E-UMTS network may be generally classified into an Evolved UTRAN (E-UTRAN) and an Evolved Packet Core (EPC). The E-UTRAN includes a User Equipment (UE); a base station (hereinafter referred to as an “eNode-B”); and an Access Gateway (AG) located at the end of the network so that it is connected to an external network.
The AG may also be referred to as an MME/UPE (Mobility Management Entity/User Plane Entity) as necessary. The AG may be classified into a traffic processing unit and a control-traffic processing unit as necessary. In this case, a first AG for processing new user traffic data may communicate with a second AG for processing control traffic data via a new interface. A single eNode-B may include at least one cell as necessary. A interface for transmitting user traffic data or control traffic data may be located between several eNode-Bs.
The EPC may include the AG; and a node for registering users of other UEs, etc. If required, an interface capable of discriminating between the E-UTRAN and the EPC may be used. S1 interface is located between the eNode-B and the AG, such that a plurality of nodes may be interconnected between the eNode-B and the AG, resulting in the implementation of Many-to-Many connection structure. One eNode-B is connected to the other eNode-B via an X2 interface. The X2 interface is always located between neighboring eNode-Bs, resulting in the implementation of a meshed network.
Radio Interface Protocol (RIP) layers located between a UE (or a terminal) and a network may be classified into a first layer (L1), a second layer (L2) and a third layer (L3) on the basis of three lower layers of an Open System Interconnection (OSI) reference model well known to those skilled in the art. A physical layer contained in the first layer (L1) provides an Information Transfer Service (ITS) over a physical channel. A Radio Resource Control (RRC) layer located at the third layer (L3) controls radio resources between the UE and the network system. For this purpose, the RRC layer allows the UE to exchange RRC messages with the network system. The RRC layer of the E-UTRAN network (or the LTE system) is located at the eNode-B.
FIG. 2 is a conceptual diagram illustrating a radio interface protocol structure between the UE and the E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) based on the 3GPP radio access network standard.
The radio interface protocol of FIG. 2 horizontally includes a physical layer, a data link layer, and a network layer. The radio interface protocol of FIG. 2 vertically includes a User Plane for transmitting data or information and a Control Plane for transmitting a control signal (also called “signaling data”).
The protocol layers shown in FIG. 2 may be classified into the first layer (L1), the second layer (L2), and the third layer (L3) on the basis of three lower layers of an Open System Interconnection (OSI) reference model well known in the art.
The above-mentioned layers of the radio-protocol control plane and the radio protocol user plane will hereinafter be described in detail.
The physical layer acting as the first layer (L1) transmits an Information Transfer Service to an upper layer over a physical channel. The physical layer is connected to a Medium Access Control (MAC) layer acting as the upper layer via a transport channel. The MAC layer communicates with the physical layer over the transport channel, such that data is communicated between the MAC layer and the physical layer. Data is communicated among different physical layers. In more detail, data is communicated between a first physical layer of a transmission end and a second physical layer of a reception end.
The MAC layer of the second layer (L2) transmits a variety of services to the RLC (Radio Link Control) layer acting as the upper layer over a logical channel. The RLC layer of the second layer (L2) supports transmission of reliable data. A variety of functions of the RLC layer may also be implemented with a function block of the MAC layer. In this case, there is no RLC layer as necessary. In order to effectively transmit IP packets (e.g., IPv4 or IPv6) within a radio-communication period having a narrow bandwidth, a PDCP layer of the second layer (L2) performs header compression to reduce the size of a relatively-large IP packet header containing unnecessary control information. The PDCP layer is located at the AG in the E-UTRAN system
The RRC (Radio Resource Control) layer located at the uppermost part of the third layer (L3) is defined by only the control plane. The RRC layer controls logical channels, transport channels, and physical channels in relation to configuration-, reconfiguration-, and release-operations of Radio Bearers (RBs). In this case, the RB is indicative of a service received from the second layer (L2) to implement data communication between the UE and the E-UTRAN.
There are downlink channels for transmitting data from the network to the UE, for example, a Broadcast Channel (BCH) for transmitting system information and a downlink Shared Channel (SCH) for transmitting user traffic data or control messages. The traffic data or control messages of a downlink multicast service or broadcast service may be transmitted over the downlink shared channel (SCH), or may also be transmitted over an additional multicast channel (MCH).
In the meantime, there are uplink channels for transmitting data from the UE to the network, for example, a Random Access Channel (RACH) and an uplink shared channel (SCH) for transmitting user traffic data or control messages.
FIG. 4 is a flow chart illustrating a handover method for controlling a UE to be handed over from a first eNode-B to a second eNode-B in the E-UTRAN system so as to continuously maintain an access state of the UE.
Referring to FIG. 4, UE context data includes area restriction information at step S401.
The UE performs a measurement process according to the measurement established in a source eNode-B, and performs a measurement control process at step S402.
The source eNode-B receives measurement information from the UE, and determines whether it will be handed over to a neighboring eNode-B (or a neighboring cell) on the basis of the received measurement information at step S403.
The source eNode-B transmits a handover (HO) request message to a target eNode-B at step S404.
The target eNode-B determines whether to receive the HO message in consideration of its own wired/wireless resources at step S405.
The target eNode-B transmits a handover (HO) response message to the source eNode-B at step S406.
The source eNode-B transmits a handover (HO) command to the UE at step S407.
Upon receiving the HO command from the source eNode-B, the UE performs a specific signaling process to connect the target eNode-B to the Layer 1 or Layer 2. The above-mentioned signaling process for the Layer 1 and the Layer 2 includes a specific process for acquiring (or gaining) synchronization between the UE and the eNode-B at step S408.
The UE connected to the Layer 1 and/or the Layer 2 transmits a handover (HO) complete message to the target eNode-B at step S409.
The target eNode-B transmits the HO complete message to the MME/UPE at step S410.
The MME/UPE transmits a handover (HO) complete acknowledgement (ACK) message to the target eNode-B at step S411.
The target eNode-B transmits a resource release message to the source eNode-B at step S412.
Upon receiving the resource release message from the target eNode-B, the source eNode-B releases all the resources at step S413.
The source eNode-B transmits the HO command to the UE, and transmits a downlink user-traffic block to the target eNode-B. In this case, the user-traffic block may be a user-traffic block transmitted from the PDCP layer of the MME/UPE, or may be a traffic block received in the RLC layer of the eNode-B such that a sequence number is added to the received traffic block. In this case, all the traffic blocks ranging from a minimum traffic block, which is incapable of completely recognizing whether the UE receives the traffic block, to the last traffic block are transmitted to the target eNode-B.
Detailed descriptions of an RRC connection process and a signal connection process will hereinafter be described.
Prior to the beginning of a call connection mode, the UE must be RRC-connected to the UTRAN, and must be signaling-connected to a CN. By means of the RRC connection and the signal connection, the UE exchanges its dedicated control information with the UTRAN or the CN.
FIG. 5 is a flow chart illustrating a method for transmitting messages exchanged between the UE and the RNC for the above-mentioned RRC connection and Initial Direct Transfer (IDT) messages for the above-mentioned signal connection.
In order to implement the above-mentioned RRC connection, the UE transmits an RRC connection request message to the RNC.
In reply to the RRC connection request message, the RNC transmits an RRC connection setup message to the UE.
The UE transmits an RRC connection setup complete message to the RNC.
If the above-mentioned processes are successfully completed, the RRC connection is implemented between the UE and the RNC.
If the RRC connection is completed, the UE transmits the IDT message, such that it begins to perform the signal connection.
A detailed description of a Random Access Channel (RACH) of a WCDMA system will hereinafter be described.
The Random Access Channel (RACH) is adapted to transmit small-sized data (i.e., short data) in an uplink direction. The random access channel (RACH) may also transmit a plurality of RRC messages, for example, an RRC connection request message, a cell update message, and a URA update message, etc.
Some logical channels (i.e., a Common Control Channel (CCCH), a Dedicated Control Channel (DCCH), and a Dedicated Traffic Channel (DTCH)) may be mapped to the radio access channel (RACH) acting as any one of transport channels. Also, the radio access channel (RACH) from among the transport channels is mapped to a Physical Random Access Channel (PRACH) acting as any one of physical channels.
FIG. 6 is a conceptual diagram illustrating operations of the physical random access channel (PRACH) according to the conventional art.
As can be seen from FIG. 6, an uplink physical channel (i.e., PRACH) includes a preamble part and a message part.
The preamble part performs a power-ramping function for suitably adjusting transport power (also called a “transmission power”) to transmit messages, and prevents several UEs from colliding with each other. The message part performs transmission of a MAC PDU message transmitted from the MAC layer to the physical channel.
The UE's MAC layer commands the physical layer of the UE to perform the PRACH transmission. Thereafter, the UE's physical layer selects a single access slot and a single signature, and transmits the PRACH preamble part in an uplink direction.
The above-mentioned preamble is transmitted to a desired destination during an access-slot time of 1.33 ms. The UE selects one of 16 signatures during an initial specific-length time of the access slot, and transmits the selected signature.
If the UE transmits the preamble part, the eNode-B transmits a response signal over a AICH (Acquisition Indicator Channel) acting as any one of downlink physical-channels. The AICH for transmitting the response signal transmits the signature selected by the preamble during an initial specific-length time of the access slot corresponding to an access slot via which the preamble is transmitted.
In this case, the eNode-B transmits an ACK (acknowledged) or NACK (non-acknowledged) message to the UE via the signature transmitted over the AICH. If the UE receives the ACK message, the UE transmits the message part of 10 ms or 20 ms using an OVSF code corresponding to the above-mentioned transmitted signature. If the UE receives the NACK message, the MAC layer of the UE commands the UE's physical layer to re-transmit the PRACH messages after the lapse of a predetermined period of time.
In the meantime, if the UE does not receive the AICH messages corresponding to the above-mentioned transmitted preamble, it transmits a new preamble at power one-step higher than that of the previous preamble after the lapse of a predetermined access slot.
FIG. 7 is a block diagram illustrating a conventional AICH structure.
The AICH acting as one of downlink physical channels will hereinafter be described with reference to FIG. 7.
The acquisition indicator channel (AICH) transmits a 16-symbol signature Si (i=0 . . . 15) during a time of an access slot corresponding to 5120 chips. In this case, the UE selects a single signature (Si) from among a plurality of signature (S0˜S15), and transmits the selected signature Si during an initial time corresponding to 4096 chips. The UE determines a specific period having the length of the remaining 1024 chips to be a transmission-power OFF period having no transmission symbols. In the meantime, similar to FIG. 4, a preamble part of PRACH acting as any one of uplink physical channels transmits the 16-symbol signature Si (i=0 . . . 15) during a time of an access slot corresponding to 4096 chips.